With multilayer piezoelectric actuators, a large displacement can be obtained but a large stress also arises in the multilayer piezoelectric body thereof. A multilayer piezoelectric actuator includes an active portion in which first and second internal electrodes, which are connected to different potentials, are stacked in a plurality of layers with piezoelectric layers therebetween; and a non-active portion positioned outside the active portion in the stacking direction. When driving is performed, a voltage is not applied to the piezoelectric layers of the non-active portion. Therefore, when driving is performed, a large stress arises between the active portion, which lengthens due to the piezoelectric effect, and the non-active portion and cracks sometimes occur in the piezoelectric body.
If such cracks progress and reach an external electrode, there is a risk of the external electrode formed on the surface of the multilayer piezoelectric body splitting. Accordingly, in order to prevent the external electrode from splitting, to date, a variety of structures have been proposed. For example, a multilayer piezoelectric actuator illustrated in FIG. 6 is disclosed in below-listed Patent Document 1. As illustrated in FIG. 6, in a multilayer piezoelectric actuator 101, a plurality of first internal electrodes 103 and a plurality of second internal electrodes 104 are alternately stacked on top of one another with piezoelectric layers therebetween in a multilayer piezoelectric body 102. The second internal electrodes 104 are led out to a side surface 102a. An external electrode 105 is formed so as to cover the side surface 102a. The external electrode 105 includes a metal base covering layer 105a formed on the side surface 102a, and an electrode member 105b that has a three-dimensional structure and is arranged on the metal base covering layer 105a. The electrode member 105b is bonded to the metal base covering layer 105a at a plurality of contact portions through conductive bonding members 106.
Even when a crack A occurs in the multilayer piezoelectric body 102 and the crack A reaches the side surface 102a, thereby splitting the metal base covering layer 105a, conduction is ensured by the electrode member 105b having a three-dimensional structure. Furthermore, the electrode member 105b, which has a three-dimensional structure, can absorb stress in the case where stress has arisen causing the crack A. Therefore, the electrode member 105b is not likely to split.
In Patent Document 1, such an electrode member 105b having a three-dimensional structure is not limited to having the shape illustrated in FIG. 6 and structures that employ a sponge metal or a metal mesh are also illustrated.
On the other hand, in below-listed Patent Document 2 and Patent Document 3, it is disclosed that a reinforcement external-electrode member, which is composed of a metal mesh, is stacked on a base electrode in a multilayer piezoelectric actuator.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-229227
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 63-153870
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-210884
As described above, a variety of external electrodes have been proposed that have a structure that absorbs stress from a multilayer piezoelectric body.
On the other hand, feeder terminals such as lead lines are bonded to the external electrodes of the multilayer piezoelectric body in order to apply a voltage for driving the multilayer piezoelectric actuator. The feeder terminals are bonded to the external electrodes by soldering, silver brazing or the like. Therefore, the bonded portions between the feeder terminals and the external electrodes do not possess elasticity. Consequently, there has been a risk of the feeder terminals becoming detached when a stress from the above-described multilayer piezoelectric body acts on the bonded portions. If the feeder terminals become detached from the external electrodes, the multilayer piezoelectric actuator can no longer be driven.
In particular, when high-speed driving is demanded, for example in the case where the multilayer piezoelectric actuator is used as a driving source for a diesel-engine fuel-injection device, it is strongly desired that the feeder terminals be securely bonded to the external electrodes. However, although to date it has been possible to design multilayer piezoelectric actuators to produce an increased displacement while being of reduced size, it has been difficult to sufficiently increase the bonding strength of the external electrodes and the feeder terminals.